Tunable optical device for dynamic chromatic dispersion and polarization mode dispersion compensation

ABSTRACT

The present invention provides a controllable double cladding guiding structure for tunable phase delay, dynamic chromatic dispersion and polarization mode dispersion compensation. The device includes an etched fiber, an electro-optic material with index of refraction changing with externally applied stimulus (electric, magnetic or thermal effect) and a fiber Bragg grating (uniform, apodized, linearly or non-linearly chirped).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/658,462 filed Sep. 10, 2003 now abandoned whichis based on U.S. Patent Application Ser. No. 60/409,197 filed Sep. 10,2002.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention describes devices for dynamic compensation ofchromatic dispersion and polarization mode dispersion providing inparticular controllable wavelength and polarization selective operation.

BACKGROUND OF THE INVENTION

Tunable filters and variable time delay (wavelength dependent delay andpolarization dependent delay) devices have a range of futureapplications as reconfigurable adding and dropping channels, dynamicflattening of the gain, polarization mode dispersion compensation anddynamic dispersion compensation on a channel per channel or on a multichannel basis.

Chromatic dispersion (CD) describes the group velocity dependence uponwavelength, i.e. different wavelengths travel at different speed alongthe fiber. CD is the sum of material dispersion and waveguidedispersion. Material dispersion is due to the fiber material's (dopedsilica) refractive index dependence upon wavelength and waveguidedispersion is defined by the refractive index profile. Dispersionimposes important limitations on high bit rate transmission in opticalfibers by broadening pulses in time domain and by distortion of thepulse shapes. At a bit rate of 40 Gb/s and beyond any slight deviationof the CD from its optimum value causes severe penalties in the system.The dispersion tolerance for high bit rates is extremely small, 30 ps/nmat 40 Gb/s. Therefore, optical communication systems will needdispersion compensators, and this is preferable on a per channel basis.The residual CD (the net amount of non-compensated dispersion at the endof the link), any slow or fast changes in the total CD in the fiberplant and in the optical components due to path changes (dynamicreconfiguration of the network), small variations in optical power,temperature fluctuations, repairs, and any other fluctuations andchanges in the fiber link will cause fluctuations in the dispersion andwill increase the bit-error rate. Therefore, the dynamic dispersioncompensation is a necessity. Furthermore, fine-tuning is necessary forhigh bit rates because the zero dispersion wavelength of the fibersvaries from one section to another.

Presently, for application at the receiver, the chromatic dispersioncompensator device can compensate either the residual dispersion aroundzero dispersion or compensate large positive dispersion of the wholesystem. Different methods in dispersion compensation technology includevirtually imaged phased array, Fabry-Perot resonator, single modedispersion compensation fiber (DCF), and high-order mode dispersioncompensation fiber. The dominant solution today is DCF. But DCF luckstunability, and has limited granularity in the amount of dispersioncompensation. It is also quite bulky, lossy, and expensive and it hassmaller field mode diameter which increases the nonlinear effects.Another solution is the fiber Bragg grating (FBG), which addresses thepresent and future challenges of dense wavelength division multiplexing(DWDM) systems. FBG's have shown a lot of promise as a potentialtechnology for tunable single channel dispersion compensation andpolarization mode dispersion devices. They also have attractivecharacteristics as low cost, simple design, low power consumption, smallsize, and good performance.

When chromatic dispersion is compensated then the polarization modedispersion (PMD) becomes one of the major obstacles for long distancetransmission at high bit rates. Polarization mode dispersion is aphysical phenomenon in optical fiber that causes light pulses to spreadin time. This may produce inter-symbol interference (two pulses overlapon one another and it will be impossible to distinguish adjacent bitsfrom each other) that will lead to an increased bit error rate (BER) atthe receiver. PMD is due to the birefringence of fibers, which arisesfrom the broken circular symmetry of the fiber by the presence of anelliptic core and from noncircular stresses. To first order, PMD may berepresented as a time delay or differential group delay between twoprincipal states of polarization (fast axis and slow axis) of theoptical fiber. PMD is a statistical process as a result of therandomness of the birefringence variations along the fiber. Thestatistical nature of PMD makes particularly difficult its control.

System PMD must be 10% of the bit period, which corresponds to 2.5 psfor 40 Gb/s transmission rates. Modern fibers have PMD values about 0.1ps/√{square root over (km)} while legacy fibers have PMD values 0.2-10ps/√{square root over (km)} resulting in PMD more than 100 ps for thetransmission distance 500 km which will lead to complete eye closureeven at 10 Gb/s transmission bit rate. We must also take into accountthe PMD of numerous optical components in the optical communicationsystem. The main problem of PMD compensation is that PMD drifts in timeand with wavelength and therefore dynamic PMD compensators are needed.

PMD compensator is composed of a polarization controller, a differentialgroup delay (DGD) device and a monitoring feedback loop. The feedbackloop is necessary to adjust the state of polarization of the incomingsignal to optimally align the DGD element. When first order PMD iscompensated the higher order PMD effects must be then compensated.

PMD can be compensated at the receiver (pre-compensation) or at thetransmitter (post-compensation). Two approaches can be used to minimizePMD. First approach is to launch the signal into a principal state ofpolarization (PSP). The second approach is to add a birefringent elementbefore the receiver to compensate the PMD. The first approach is slowbecause of the transmission of the feedback signal from the receiver tothe transmitter. For example for a 500 km link it takes 2.5 ms for afeedback signal from transmitter to receiver. Therefore the performanceof the PMD compensator at the transmitter is limited.

PMD compensators were demonstrated using a variable DGD such as a freespace polarization delay line. The free space polarization delay line isachieved by bringing light out of fiber, physically separating twoorthogonal polarizations with a polarization beam splitter andrecombining them with a polarization beam combiner. Such an out-of fiberdevice has high losses, large output polarization fluctuation, largefootprint, poor control certainty due to mechanical motion, and it haslow speed. Another technology is to use as DGD a birefringent in-fiberelement such as a highly birefrengent nonlinearly chirped fiber Bragggrating. This technology has a potential promise as a birefringentelement for PMD compensation. Several schemes for obtaining tunablechirp in fiber gratings written in polarization maintaining fiber wereproposed: either by uniform tuning of a nonlinearly chirped fiber Bragggrating or by application of a nonuniform external gradient, such as atemperature or strain gradient. See U.S. Pat. No. 5,982,963 (Feng et.Al). These mechanisms of FBG tuning are highly temperature sensitive andpower consuming both of which are undesirable, particularly in compactintegrated geometries.

Accordingly, we propose an efficient in-fiber optical device based onFBG (uniform or apodized or linearly chirped or non-linearly chirped)for dynamic chromatic dispersion and polarization mode dispersioncompensation (PMDC).

SUMMARY OF THE INVENTION

The techniques and devices of this application include optical devicesthat can produce phase delay, variable time delay between two principalstates of polarization and a mechanism to dynamically adjust thedispersion of FBG that is initially uniform or apodized or linearly ornonlinearly chirped. The present invention comprises an etched fibercontaining a FBG, which is surrounded by an electro-optic materialapplied in the proximity of the grating. The electro-optic material is acomposite polymer liquid crystal (CPLC) that has a low ordinaryrefractive index n₀. The dynamically tuning device is realized bycontrolling the refractive index of the CPLC material. The refractiveindex of the CPLC can be changed applying the external stimulus(electrical, magnetic or thermal). The variation in the properties ofthe grating (chirp) may be determined by the original chirp or byapplying a stimulus gradient to the CPLC material. The chirp and centerwavelength can be controlled independently by the magnitude of theapplied voltage and its gradient using multi-electrode geometry.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a schematic illustration of operation conditions of anembodiment of the present invention implemented as a variable phasedelay and tunable filter;

FIG. 2 is a schematic external illustration of an optical device toobtain a variable phase delay;

FIG. 3 is a chart showing the calculated change of the dispersion(refractive index dependence of the core mode upon wavelength) due tothe applied voltage in the system of FIG. 2;

FIG. 4 schematically illustrates the operation of an embodiment of thepresent invention implemented as a variable wavelength selective andpolarization selective delay;

FIGS. 5 a & 5 b schematically illustrate possible variations of theembodiment of FIG. 4 using multiple electrodes;

FIG. 6 is a chart showing the calculated shift of the reflectionspectrum in the system of FIG. 4;

FIG. 7 is a chart showing the calculated wavelength shift of thereflection spectrum due to the applied stimulus in the system of FIG. 4;

FIG. 8 is a chart showing calculated shift of the reflection spectrum inthe system of FIG. 5;

FIG. 9 is a chart showing time delays of reflected signals as a functionof wavelengths that are calculated in the fiber gratings of FIG. 5;

FIG. 10 is a chart showing nonlinear time delays of reflected signals asa function of wavelengths that are calculated in the fiber gratings ofFIG. 5;

FIGS. 11 a & 11 b are the diagrams of implementations of the system ofFIGS. 4 & 5 for dynamic dispersion compensation;

FIG. 12 is a geometrical diagram of one implementation of the system ofFIGS. 4 & 5 as a variable delay compensator;

FIG. 13 is a diagram showing a system for polarization mode dispersioncompensation using a birefringent CPLC material;

FIGS. 14 a & 14 b are the charts showing the calculated time delays fortwo orthogonal states of polarization as a function of wavelength andthe respective nonlinear dependence of the differential time delay uponthe wavelength for the system of FIG. 4 and FIG. 5;

FIGS. 15 a & 15 b is a diagram of one implementation of the system ofFIGS. 2, 4 &5 for polarization mode dispersion compensation;

FIG. 16 shows a schematic illustration of an optical device to obtain atunable narrowband transmission filter;

FIGS. 17 a & 17 b are charts showing the narrowband transmission shiftdue to the locally applied voltage in the system of FIG. 16;

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Further discussion will use the example of circular waveguides (fibers)while planar waveguides can also be used. An optical fiber is a smalldiameter waveguide which has an axi-symmetrical core and a cladding. Oneaspect of the techniques of this disclosure is to use a double claddingstructure when a FBG (uniform or apodized or linearly or nonlinearlychirped) is formed in the core of the fiber to obtain a dynamicallyadjustable chromatic dispersion and dynamically adjustable time delaybetween two orthogonal polarization components. Other kind of gratingsmay be also used.

In this disclosure the evanescent field approach was used. It is wellknown that the cladding contributes largely to the power propagation inthe fiber. Thus the guidance in the fiber is affected by the refractiveindex of the surrounding medium if it is placed in the vicinity of thefiber core so that the evanescent field of the guided light can “see”the external index of refraction n₃ (See FIG. 1). Therefore any changesin the refractive index of the external material will affect theeffective index of refraction and the propagation properties of theguided light.

Our approach is based on placing a birefringent electro-optic material,composite polymer liquid crystal (CPLC), in the vicinity of theevanescent field of the guided mode that allow the electro-opticmodifications of the refractive index of surrounding medium n₃ changethe effective index of refraction of the core n_(eff). This structure iscomposed of the core (with the refractive index n₁), ultra thin cladding(with the refractive index n₂) and surrounding medium (with therefractive index n₃) and it can be considered as a three-layer structure(doubly clad fiber) in which third layer structure has a dynamicallycontrollable refractive index n₃ (see FIG. 1). Further we will call itthe external index of refraction.

To approach to the evanescent field of the core mode the thickness ofthe cladding was reduced by chemical etching using hydrofluoric (HF)acid solution. The thickness of the cladding was controlled by theconcentration of solution and by the etching time. The etched fiber isinserted between two glass plates separated by a gap of 15-30 μm. Theinner surfaces of glass plates are deposited with transparent electrodes(indium tin oxide). The system is treated to ensure that the liquidcrystal molecules are initially oriented parallel to the direction offiber axis. The space between glasses is filled with a CPLC material(see FIG. 2). Because of orientational ordering of rodlike molecules,nematic liquid crystal is uniaxially symmetric (with two principalrefractive indices n_(o) and n_(e)) with the axis of symmetry parallelto average orientation of the axes of molecules (director). The ordinaryrefractive index n_(o) corresponds to light with electric fieldpolarization perpendicular to the director and extraordinary refractiveindex n_(e) for the light with electrical field polarization parallel tothe director. Applying external voltages one can control the orderingand the orientation of liquid crystal molecules and therefore theirrefractive index. The birefringence of composite liquid crystalmaterials can reach rather high values Δn=n_(e)−n₀≈0.2. To ensure thatthe core mode is guided, the refractive index of the electro-opticmaterial must be always less than the effective refractive index of thecore mode n_(eff). For practical applications the refractive index ofCPLC should always stay lower than the refractive index of the claddingmaterial. There are several different molecular orientationconfigurations that can be applied to this device (see, for example,Applicant's co-pending U.S. patent application Ser. No. 10/237,622). Wewill consider one of these geometries as an example. In the initialstate the CPLC molecules are aligned parallel to the fiber and to thesurfaces of the electrode plates. When a voltage is applied to theelectrodes the CPLC molecules are forced to align parallel to theelectric field.

The devices schematically shown in FIGS. 2, 4&5 with unmodified corerelies either on single pair of electrodes or on multiple electrodegeometry. A composite polymer liquid crystal material is disposed aroundthe cladding within the control region and is capable of interactionwith the evanescent field of the core mode. A controller is arranged toexternally apply the stimulus to the CPLC material.

FIG. 3 shows the effective index change with wavelength (dispersion) fortwo values of the external index of refraction. The first curve of FIG.3 was obtained for n2=n3=1.444, r2=r1=1.75 μm and the second curve forn2=1.444, n3=1.434, r2=r1*1.2 and r1=1.75 μm (see FIG. 1). The length ofthe controllable region can vary from several mm to several centimetersduring fabrication. For example for the wavelength λ=1.5 μm we obtain achange in the effective index of refraction Δn_(eff)=1.1*10⁻³. Thisvalue will lead to a large phase delay (up to several π) between twoorthogonal polarizations for the system shown in FIG. 2. It can be usedas a polarization controller for polarization mode dispersioncompensation. Another aspect of this index change is the wavelengthshift of about 1 nm for a typical uniform or chirped fiber Bragg gratingoperating in telecom wavelength range (see FIG. 4).

FIG. 4 illustrates a block diagram showing the structure of a dynamicdispersion compensation device. First a fiber Bragg grating was formedin the core of the fiber. The grating is formed only in the core if onlythe core is doped, otherwise, it could be formed also in the cladding.

The chirped fiber Bragg gratings can be realized either by varying theperiod of the grating along the grating or the effective index ofrefraction of the core mode. Bragg phase matching condition is given by:λ_(B)(z)=2n _(eff)(z)·Λ(z)  (1)

where n_(eff) and Λ are the effective refractive index of the core andthe period of the grating, respectively. They may be changed uniformly,linearly or nonlinearly along the length of a grating.

For chirped gratings the Bragg wavelength varies along the length of thegrating. Therefore the different wavelength components of the signal arereflected from different locations or at different times along the Bragggrating. The shorter wavelengths are reflected at the near end ofgrating and the longer wavelengths at the far end or vice versa(depending on the sign of the chirp). Thus the longer wavelength isexperiencing an additional time delay with respect to shorterwavelengths or vice versa. Therefore chirped FBG's can be used as adispersion compensation device.

The chirp can be obtained by chirping the effective index of refractionn_(eff), period Λ or combination of both of them. For example, U.S. Pat.No. 5,007,705 (Morey et al.) teaches a tunable FBG in which a heatingelectrode is used to change the geometrical period Λ_(FBG) of thegrating or the refractive index n_(eff)^(core)of the core material.

U.S. Pat. No. 5,982,963 (Feng et al.) teaches that a nonlinearly chirpedfiber Bragg grating can be used as a dynamic dispersion compensator bystretching the fiber or by applying an electromagnetic radiation.

In the present disclosure the initially formed FBG may be uniform orchirped (see FIGS. 4 &5). The change in Bragg wavelength along thefiber, which defines the chirp, is determined by externally appliedstimulus (uniform voltage along the grating for nonlinearly chirpedFBG's or voltage gradient for uniform FBG's). The change in Braggwavelength is related to the change in the effective refractive index ofthe core. For example in order to induce a tunable chirp, the voltagemust be applied in such a way that the change in the effective indexvaries linearly or nonlinearly with position along the grating. Here wedescribe a device capable of producing different chirp profiles in acontrolled manner and the possibility of tuning the wavelength resonanceof fiber Bragg gratings. Our solution is based on either using initiallynonlinearly chirped FBG and then changing uniformly the effectiverefractive index or using initially non-chirped FBG and then changingnonlinearly the effective index of refraction based on multipleelectrode geometry. By choosing appropriate multi electrode geometry andapplying voltage gradient we can independently control the chirp and thewavelength shift (see FIG. 5).

FIG. 6 shows the calculated shifts of reflection spectrum due to thechange in the refraction index of the CPLC for three values of therefractive index of the composite liquid crystal n2=n3=1.444,n2−n3=0.01, and n2−n3=0.02 for linearly chirped FBG Bragg gratings withapproximately 1 nm bandwidth and r2=r1*1.2.

FIG. 7 shows the calculated shift of the Bragg wavelength in thereflection spectrum as a function of the external index of refraction.The wavelength shift is not linear but rather smooth with the externalindex change (proportional to the applied voltage). For external indexchange n₂−n₃=0.025 (which is possible in special fibers), the reflectionpeak shifts by 1.8 nm. A gradient in the applied voltage along the fibercauses a chirp in the grating. The approximately linear external indexgradient (which depends on the applied voltage) induces a similargradient in the effective index of refraction. This method allows us toobtain the nonlinear chirps of complex form in the effective index ofrefraction of the core mode. The change in the Bragg wavelength alongthe fiber, which defines the chirp, is related to the etched diameter(defined during fabrication), applied voltage and the geometry ofelectrodes. In order to induce a nonlinear chirp we must apply thevoltage gradient nonlinearly along the fiber (FIG. 5). For linearlychirped FBG's to tune the dispersion we must tune the chirp. Fornonlinearly chirped FBG's we will uniformly tune the effective index ofrefraction along the grating. Therefore, we will use the technique wherethe dispersion is tuned by varying the magnitude of the voltage gradientor by changing the chirp rate.

FIGS. 8&9 show the ability to tune the dispersion by adjustingexternally the chirp C=−1*10⁻¹¹ 1/nm (curve 1), C=−2*10⁻¹¹ 1/nm (curve2), and C=−3*10⁻¹¹ 1/nm (curve 3). The reflection spectrum is broadeningdue to a change of the chirp. The reflectivity was maintained above 95%for all values of the chirp. The reflection spectrum of the gratingshifts to a longer wavelength, the maximum reflectivity decreases andstrong amplitude ripples appear. FIG. 9 shows the respective group delayvalues. The dispersion values in FIG. 9 are 220 ps/nm (curve 3), 350ps/nm (curve 2) and 910 ps/nm (curve 1) for chirp C=−3*10⁻¹¹ 1/nm,C=−2*10⁻¹¹ 1/nm, and C=−1*10⁻¹¹ 1/nm, respectively. Time delay is alinear function with wavelength because different wavelengths arereflected at different points of the grating. As we can see by changingthe linear chirp we can tune the dispersion. In order to do not shiftthe reflection band we can tune simultaneously the chirp and peakwavelength.

Another aspect of the present invention is a tuning of the nonlinearfiber Bragg grating. Delay for nonlinearly chirped FBG (quadraticchirped grating) changing quadratically (see FIG. 10). The chirpC=−1*10⁻¹¹ 1/nm (curve 1), C=−2*10⁻¹¹ 1/nm (curve 2), and C=−3*10⁻¹¹(curve 3). The dispersion can be easily tuned (by more than 1000 ps/nm)only by uniformly changing the effective index of refraction (in givenexample it changes by more than 400 ps/nm). The reflectivity ofquadratic chirped gratings is not uniform along the reflection band. Ithas the characteristics of a uniform grating at wavelengths where chirpis small and reflectivity decreases at wavelength where chirp has largevalues.

FIG. 11 shows the typical system having a dispersion compensationelement. FBG is connected to the fiber link by means of the circulator.From FIG. 11 we see that the pulse is continuously monitored tocalculate the dynamic dispersion and the FBG's parameters are updatedcontinuously or dynamically. After the circulator a feedback loop isnecessary to tune the grating and thereby to dynamically track changesin dispersion of the fiber. FIG. 11 a demonstrates a single channeldynamic dispersion compensator. Multi channel dynamic dispersioncompensator is also possible (see FIG. 11 b).

Differential group delay (DGD) is the time domain manifestation ofpolarization mode dispersion (PMD). There is also frequency domainmanifestation of PMD due to the optical birefringence dependence uponfrequency Δn(ω) or wavelength Δn(λ). The output polarization from thefiber undergoes a rotation on the Poincare sphere. The three-dimensionalrotation vector {right arrow over (Ω)} describes the rate of rotationand is called a dispersion vector. The magnitude of this vector is DGD:|{right arrow over (Ω)}|=Δτ. The direction of the vector defines an axiswhose two intercepts with the surface of Poincare sphere correspond tothe two principal states of polarization (PSP) of the fiber output. If aPSP is excited at the input of the fiber then pulses will propagateundistorted. Therefore the signal will stay completely polarized and theoutput polarization remains constant to first order as a function ofoptical frequency. If the signal polarization is a mixture between PSP'sthen the pulse broadens in the time domain and the signal becomesdepolarized in the frequency domain.

Higher order PMD describes the change in the PMD vector both inmagnitude and in direction with changes in frequency. Second order PMDis described by the derivative of {right arrow over (Ω)}(ω) and can berepresented mathematically as: $\begin{matrix}{{\overset{\rightarrow}{\Omega}}_{\omega} = {\frac{\mathbb{d}{\overset{\rightarrow}{\Omega}(\omega)}}{\mathbb{d}\omega} = {{{\Delta\tau}_{\omega} \cdot \overset{\rightarrow}{q}} + {\Delta\quad{\tau \cdot {\overset{\rightarrow}{q}}_{\omega}}}}}} & (2)\end{matrix}$

The first term, which is parallel to {right arrow over (Ω)}, representsthe change of DGD with frequency and is frequently called thepolarization dependent chromatic dispersion, and the second term, whichis orthogonal to {right arrow over (Ω)}, represents the rotation of theprincipal state of polarization and is called the depolarizationcomponent.

The basic concept of the variable delay compensator is shown in FIG. 12using the geometrical representation of the PMD vector. The total PMDvector can be cancelled or aligned along the state of polarization.

 |{right arrow over (Ω)}_(tot)(ω)|=|{right arrow over (Ω)}(ω)+{rightarrow over (Ω)}_(comp)|  (3)

where {right arrow over (Ω)}_(comp) is the compensating vector. Byapplying an opposite {right arrow over (Ω)}_(comp) at the receiver wecan compensate first order PMD of the system. The second order PMD isapproximately perpendicular to {right arrow over (Ω)} of the system.Therefore aligning the input state of polarization (SOP) of the signalwith the derivate {right arrow over (Ω)}_(ω) we can compensate thesecond order PMD.

U.S. Pat. No. 5,473,457 (Ono) teaches that analyzing a received opticalsignal and separating the received pulse into fast and slow modes onecan compensate PMD. Fast mode components are delayed by a polarizationmaintaining fiber.

Variable DGD compensator provides superior performance in a system witha dynamic tracking control. A technique of using a double stagecompensators to compensate also for higher order PMD is disclosed by Yuet al. in “Higher order polarization mode disprsion compensation using afixed time delay followed by a variable time delay”, IEEE PhotonicsTechnology Letters, Vol. 13, No. 8, P. 863-865 (2001). The first stagewith a fixed DGD is used to compensate the second order PMD. The secondstage is used to compensate the residual first order PMD.

U.S. Pat. No. 6,330,383 (Cai et al.) teaches that a nonlinearly chirpedfiber Bragg grating, when formed in an optical birefringent material(polarization maintaining fiber), may be used to produce a dynamicallyadjustable time delay between orthogonal polarizations. Anonlinearly-chirped fiber Bragg grating written into ahigh-birefringence fiber can be used as a variable DGD for PMDcompensator. The high-birefringence fiber provides a different delay fordifferent SOPs, and the nonlinear chirp provides the ability for tuningof the specific amounts of time delay between two PSP. The differentialtime delay can be tuned by using thermo-optic, piezoelectric,acousto-optic effects, fiber stressing, compressing or bendingmechanisms.

In the present disclosure the variable DGD is obtained using the systemof FIG. 1. By selecting and controlling the variable-index birefringentmaterial such that the refractive index of the variable index materialalways stays less than the effective refractive index n_(eff) of thecore modes.

FIG. 13 illustrates a FBG formed in the core and a birefringent CPLCthat has high refractive index difference between fast and slowpolarization axes. The differential time delay between two PSP isdefined by the difference in reflection points. The difference inreflection points ΔL causes a differential time delay Δτ between twoPSP. Initially both orthogonal polarizations will “see” the sameexternal index of refraction n₀. As the refractive index of the variableindex material increases (staying below n_(eff)), light with electricfield polarization in the same plane as the director will “see” a higherindex of refraction than the light with electric field polarizationperpendicular to the director which will “see” the unperturbatedeffective index of refraction n₀. The value of the time delay betweentwo orthogonal polarizations is controlled by the difference between theexternal index of refraction and the effective index of refraction ofthe core mode n_(eff). It also depends on the chirp parameter, which wecan control by applying the stimulus gradient for the given length ofthe FBG. FIG. 14 a shows the calculated time delay dependence onwavelength for two orthogonal states of polarization (see FIG. 13) fornonlinearly chirped FBG with bandwidth of about 1 nm at 1550 nm andlength L=10 cm. The slow axis corresponds when the external index ofrefraction n₃=n₂=1.444, and fast axis corresponds when n₂−n₃=0.02. FIG.14 b shows the respective nonlinear dependence of the time delay on thewavelength that changes from about 160 ps to 350 ps within 4 nmbandwidth.

FIG. 15 shows a typical double stage PMD compensation system where acirculator, or other similar methods to combine optical signals withoutinterference because they have orthogonal polarizations. The systemshown in FIG. 2 can be used as a polarization controller and the systemshown in FIGS. 4&5 can be used as a fixed or differential polarizationdelay element (see FIG. 15 b). Another possibility is to change thesystem of PC and DGD element by only one system but withmulti-electrodes cylindrical geometry in order to dynamically change thebirefringence axis or director.

FBG's are in general acting in reflection geometry. A bandpasstransmission response is advantageous for many applications, for exampleas a individual channel selection in DWDM systems. It also avoids theneed of circulators, couplers or other similar optical components toseparate the reflected signal from the input signal.

In the present disclosure we present a tunable narrowband FBG filter. Toobtain a tunable phase shift we will use the system shown in FIG. 16.Tuning of the transmission resonance peak can be achieved for ourtechnology simply changing the effective index of refraction in thecenter of the grating (see FIG. 3). For example to obtain a π/2 (or λ/4)phase shift the length of the electrodes should be L=400 μm. A phaseshift in the grating results in a narrow transmission window within thestop band of the grating. Phase shifted Bragg gratings are shown inFIGS. 17 a and 17 b (for phase shift π/2 and 2π/3, respectively). Thecalculated reflectivity is shown for uniform Bragg grating. This methodmay be applied to obtain tunable phase shifts in other kind of gratings(apodized, non-uniform) which is rather difficult to obtain even in thestatic regime.

Another possibility to obtain a tunable phase shift is based on cleavingthe fiber within FBG, and filling the gap between the two parts of theFBG with controllable refractive index material such as an electro-opticmaterial, thermo-optic material or other.

The embodiments of the invention described above are intended to beexemplary only. The scope of the invention is therefore intended to belimited solely by the scope of the appended claims.

1. A method for controlling dispersion of light propagating within anoptical waveguide comprising a core surrounded by a cladding having asubstantially fixed index of refraction, the method comprising a stepof: controlling a differential group delay of light reflected by agrating within the waveguide by varying a refractive index of avariable-index material surrounding the cladding at least in a controlregion of the waveguide proximal to the grating and having a claddingthickness which is less than a penetration depth of an evanescent fieldof light propagating in the grating.
 2. A method as claimed in claim 1,wherein the grating is chirped, and wherein the step of controlling thegroup delay of light reflected by the grating comprises a step ofapplying a voltage to the variable index material, a magnitude of thevoltage being substantially uniform along the length of the grating. 3.A method as claimed in claim 1, wherein the grating is uniform, andwherein the step of controlling the group delay of light reflected bythe grating comprises a step of applying a voltage to the variable indexmaterial, a magnitude of the voltage defining a voltage gradient alongthe length of the grating.
 4. A method as claimed in claim 1, whereinthe variable index material is a birefringent material.
 5. An opticaldevice for controlling dispersion of light propagating within an opticalwaveguide comprising a core surrounded by a cladding having asubstantially fixed index of refraction, the optical device comprising:a control region of the optical waveguide in which a thickness of thecladding is less than a penetration depth of an evanescent field oflight propagating in the waveguide core; a grating within the core ofthe control region; a variable-index material surrounding the claddingat least in the vicinity of the grating, the variable-index materialhaving an index of refraction that is controllable in response to anapplied stimulus; and a controller adapted to control a differentialgroup delay of light reflected by the grating by controllably applyingthe stimulus to the variable-index material at least in the vicinity ofthe grating.
 6. An optical device as claimed in claim 5, wherein thegrating extends beyond an end of the control region.
 7. An opticaldevice as claimed in claim 5, wherein the variable index material is abirefringent material.
 8. An optical device as claimed in claim 5,wherein the variable-index material is an electro-optic materialresponsive to an applied voltage, and wherein the controller comprises:at least one pair of electrodes in electrical contact with the variableindex material and disposed on opposite sides of the waveguide; and avoltage source for applying a selected voltage across the pair ofelectrodes.
 9. An optical device as claimed in claim 8, wherein thegrating is chirped, and wherein a single pair of electrodes extend alongat least a portion of the grating such that a substantially uniformvoltage is applied along the grating.
 10. An optical device as claimedin claim 8, wherein the grating is uniform, and wherein there are atleast two pairs of electrodes disposed along the length of the grating,each pair of electrodes being connected to the voltage source to receiverespective different voltages, so as to produce a voltage gradient alongthe grating.